The present invention relates to instruments including a probe for measuring electrical voltages and more particularly concerns an electronic amplifier circuit for such a probe, which imparts thereto a high input impedance so as to minimise the interference produced by the probe when measuring continuous or variable signals at a very low power level.
Measuring operations of this kind are generally carried out for example in biology and in chemistry and indeed in tests on integrated circuits.
The essential characteristic of a probe for performing such measurement operations, whether the probe is passive or active, is given by its input impedance which must be as high as possible. The input impedance of the probe may be represented to a high degree of approximation by a resistor connected in parallel with a capacitor. The best performances are attained by probes which contain active devices such as transistors as they have a high input impedance and as they make it possible to produce output signals which are substantially of the same magnitude as the signals being measured. The circuit for a probe, in accordance with the present invention, falls into that category.
The active devices which are available at the present time, for example field effect or MOS transistors, make it possible to give very high input resistance values. On the other hand, the parasitic capacitances associated with such devices are not always negligible and, unless precautions are taken, the influence of such capacitances may give rise to substantial error, in particular when measuring high-frequency voltages.
The method which is generally employed for attenuating the effect of an element (for example a resistor or a capacitor) on a circuit is a feedback control, which comprises applying to one terminal of the element, a voltage which is substantially equal to the voltage at its other terminal. In fact, if that voltage difference is zero, the current passing through the element will also be zero and the circuit will behave as if that element had infinite impedance.
Circuits based on that principle have long been known and produce a good performance. The literature includes for example the article by D. P. Stokesberry (Proc. of the IEEE, Jan. 1966, page 66) which describes a simple circuit for a probe, using two discrete transistors and a Darlington amplifier. Proper operation of that circuit requires the voltage gain of the emitter-followers to be close to unity, which requires a substantial bias current for the transistors and high-resistance load resistors. This means that the supply to that circuit requires a high voltage, 75 volts in this case, which results in a high level of heat dissipation. This is obviously a serious limitation in regard to use of such a circuit and also makes it unsuitable for production in integrated form.
Another circuit which does not suffer from these deficiencies has been more recently described by H. J. De Man et al (IEEE J. Solid-State Circuits, vol. SC-12, No 3, June 1977). This circuit is in the form of an integrated circuit and the supply voltage required is only 18 volts. The characteristics of the circuit are very good since it has an input resistance of more than 1000 M.OMEGA. and an input capacitance of the order of 0.1 pF. However, this circuit is complicated in structure, based on using a differential amplifier. It comprises about ten transistors, and many other components.